p-Hydroxymethylbenzoic acid (pHMBA) is an important monomer for preparation of poly(p-methylenebenzoate). Poly(p-methylenebenzoate) is prepared from pHMBA by polymerization under polycondensation and melt polymerization conditions in the presence of a suitable catalyst.
Numerous methods are known for the preparation of p-hydroxymethylbenzoic acid. Among other methods, some of these are based on the saponification of a corresponding halogenmethyl compound, such as p-chloromethylbenzoic acid or the esters thereof or p-chloromethylbenzonitrile. For example, several methods for the synthesis of p-hydroxymethylbenzoic acid are taught in U.S. Pat. No. 4,130,719. The electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid is taught in commonly-assigned application Ser. No. 319,120, filed Nov. 9, 1981, now U.S. Pat. No. 4,381,229.
p-Hydroxymethylbenzoic acid must be free from by-products when it is to be employed in polycondensation reactions, such as in the preparation of polyesters. However, most of the known processes for the preparation of p-hydroxymethylbenzoic acid do not yield the acid free from by-products. Thus, for example, during the saponification of highly pure p-chloromethylbenzoic acid in a faintly alkaline aqueous medium, up to 10% of diben- zylether-4,4'-dicarboxylic acid is always produced.
In other methods, as for example, in the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid many competing reactions take place in the electrolysis cell. The resulting presence of 4-carboxybenzaldehyde (4-CBA), dihydroxymethylbenzene, toluic acid and other impurities render the resulting p-hydroxymethylbenzoic acid undesirable for use as a monomer for polymer applications without further expensive purification.
It is well-known that in the cathodic reduction of carboxylic acids that two types of products can result, either the corresponding aldehyde in a two-electron process or the hydroxymethyl compound in a four-electron process where the aldehyde is further reduced to the alcohol. (M. Baizer, Organic Electrochemistry, Deker, N.Y. (1973), 414) The alcohol can be further reduced to the methyl group.
4-Carboxybenzaldehyde and p-toluic acid, both of which occur in the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid, and residual terephthalic acid act as polymer chain stoppers in polymerization of p-hydroxymethylbenzoic acid to poly(p- methylenebenzoate).
From mathematical calculations, a combined level of above 0.3 (wt)% of monocarboxylic acid impurities, i.e., 4-carboxybenzaldehyde and p-toluic acid, will limit molecular weight of the polymer chain and give a polymer with inferior mechanical properties and an inherent viscosity of less than about 0.6 deciliters/gram (dl/g) in a 60/40 phenol/tetrachloroethane solvent at 30.degree. C. An inherent viscosity of at least 0.6 dl/g is suitable for preparation of molded parts having a tensile impact strength of at least 100 psi, according to ASTM D-1822, and for preparation of fibers and films of poly(p-methylenebenzate).
Although residual terephthalic acid impurities in p-hydroxymethylbenzoic acid create a stoichiometric imbalance of hydroxyl and carboxylic acid units such that the resultant polymer has a predominance of carboxylic acid end groups, terephthalic acid can be incorporated into any location of the polymer chain with consequent limited weight development. As a result, higher levels of residual terephthalic acid impurity can be tolerated than levels of 4-carboxybenzaldehyde and p-toluic acid.
4-Carboxybenzaldehyde is a particularly undesirable impurity because it acts as a chain-stopper during polyesterification and can be present in significant quantities, in a ratio of about 2:1, to p-toluic acid, as taught in U.S. Pat. No. 3,850,983. Although 4-carboxybenzaldehyde is difficult to remove by physical means, it can be hydrogenated to toluic acid and other derivatives, but toluic acid also acts as a chain-stopper during polyesterification. Toluic acid can be efficiently removed by cooling and crystallizing crude p-hydroxymethylbenzoic acids containing it. 4-Carboxybenzaldehyde also can be reduced to the hydroxymethyl compound, i.e., p-hydroxymethylbenzoic acid, in an electrochemical process as is taught by Baizer, mentioned above, but reduction of terephthalic acid in an electrochemical process results in increased quantities of 4-carboxybenzaldehyde despite the concurrent hydrogenation of 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid.
Catalytic methods of hydrogenating 4-carboxybenzaldehyde typically result in the production of toluic acid. For example, in the purification of terephthalic acid containing 4-carboxybenzaldehyde by catalytic hydrogenation, U.S. Pat. No. 4,260,817 teaches that hydrogenation of terephthalic acid using a bimetallic catalyst of palladium and platinum converts 4-carboxybenzaldehyde to p-toluic acid.
The presence of impurities which can act as color bodies is also highly undesirable. Color may appear in the newly-manufactured p-hydroxymethylbenzoic acid or can develop on standing or exposure to elevated temperatures or actinic light. Such color-forming impurities are occluded in the acid crystals and carry over into resins made from the acids, reducing or obliterating the market value of the end product. In uses which require less attention to color, the fact that the product is white or near white will still render the product more desirable. The process of this invention is useful in reducing the presence of impurities which can affect the color of the acid produced thereby and consequently the color of the final resin.
A batch or continuous process has now been found for the hydrogenation of 4-carboxybenzaldehyde in the presence of p-hydroxymethylbenzoic acid wherein one aldehyde group is hydrogenated to a hydroxymethyl group using a nickel catalyst and yet is selective enough to prevent hydrogenolysis of the hydroxymethyl group of the p-hydroxymethylbenzoic acid to the methyl group and reduction of the second carboxylic acid to a hydroxymethyl group or to a methyl group. Potential color bodies can also be reduced. More specifically, a process has been found for the preparation of p-hydroxymethylbenzoic acid by the reduction of 4-carboxybenzaldehyde and potential color bodies in the presence of a nickel catalyst under relatively mild conditions wherein conversion of p-hydroxymethylbenzoic acid is minimal and selectivity to p-hydroxymethylbenzoic acid is within the range of from about 80% to 90%. Yields are accordingly within the range of about 90 (wt)% of 4-carboxybenzaldehyde present.
Unexpectedly, it has been found that a nickel catalyst has catalytic activity in aqueous solvent to selectively hydrogenate an ammonium or alkali metal or alkaline earth metal salt of 4-carboxybenzaldehyde to the equivalent salt of p-hydroxymethylbenzoic acid in yields of 90 (wt)% or better. Only the aldehyde group is reduced to the hydroxymethyl group.
Color level of the purified salt of p-hydroxymethylbenzoic acid in batch or continuous process can be controlled effectively by modulating the hydrogen concentration in the impure solution while it is undergoing hydrogenation, in the method of U.S. Pat. No. 4,626,598, which is incorporated by reference.
Accordingly, it is an object of this invention to purify p-hydroxymethylbenzoic acid containing 4-carboxybenzaldehyde by hydrogenating 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid without increasing the level of p-toluic acid present.
It is a further object of this invention to hydrogenate 4-carboxybenzaldehyde to p-hydroxymethylbenzoic acid, in the presence of p-hydroxymethylbenzoic acid wherein over-hydrogenation is controlled, as over-hydrogenation can produce not only p-toluic acid but also a number of hydrogenated aromatic products including cyclohexane and several hydrogenated aromatic acids such as 1,4 cyclohexanedicarboxylic acid.
It is a further object of this invention to prepare p-hydroxymethylbenzoic acid in a highly purified state.
It is a further object of this invention to prepare p-hydroxymethylbenzoic acid in a highly-purified state for polymerization to poly(p-methylenebenzoate), the said poly(p-methylenebenzoate) having an inherent viscosity of greater than 0.6 dl/g in a 60/40 phenol/tetrachloroethane solvent at 30.degree. C.